Giovanni Trapattoni has criticised Robbie Keane for playing on with a muscular tear against Andorra last Friday, which he feels made the injury worse and looks to have ruled the Republic of Ireland captain out of the Euro 2012 play-off next month.

Keane admitted that he aggravated the problem in the back of his thigh, with which he had reported for international duty, when he pulled the ball back for Kevin Doyle to open the scoring in the seventh minute against Andorra. He went on to complete the full 90 minutes in the 2-0 victory.

The 31-year-old sat out the 2-1 home win over Armenia on Tuesday, which secured the team's place in the play‑offs, the draw for which takes place on Thursday. Ireland have seeded status and will face Turkey, Bosnia-Herzegovina, Montenegro or Estonia.

Trapattoni has expressed the hope that Keane will be out for a month and therefore could be available, with the help of a pain-killing injection, for the ties. The first leg is scheduled for 11 or 12 November, with the return on 15 November. But Keane has painted a bleaker picture by suggesting that the tear is sufficiently serious to keep him out for "four to six weeks".

The Italian was asked whether he was annoyed that Keane had not signalled to him the moment that he felt the injury, which might have allowed him to be substituted and possibly preserved. "Sure, I said this to him after," Trapattoni said. "If it was a knock, I am not afraid but he made it worse [by playing on].

"I said to him that he should have said immediately and maybe go off. It should be immediately with muscle [problems]. Never discuss, never put into question the muscular problems. He was a doubt before the game with it. Sure, you can never play 90 minutes with this injury. But sometimes, it's just the eagerness, the willingness to play."

Keane, who consulted a specialist in Ireland on Monday and will continue his rehabilitation at his club, the Los Angeles Galaxy, faces an anxious wait. "We'll know more in 10 days," the striker said. "I've got to get another scan and hopefully it settles down and isn't as bad as it was when I had the first scan done immediately after the game, when there was a lot of bleeding. I'll go back to the club and see. It's a four- to six-week injury and it will be touch and go."

Ireland endured an arduous journey back to Dublin after the Andorra game and, with a delay to the flight, they did not arrive until 7am on Saturday. It has been suggested that this served to deepen Keane's discomfort. "No, that's not true … it was just one of those things," he said. "I actually did it during Kevin's goal. The adrenaline gets you through but straight after the game, I knew that it was a tear.

"Of course, I would take injections [but] this is different. It's a muscle injury and not just pain. It's a grade-two tear, grade three is the worst. It's disappointing and I'll do my best to get back. The medical facilities are very, very good in LA and hopefully the doctors over there will get me back."

Keane's possible absence has been compounded by Doyle's suspension from the first leg; the Wolverhampton Wanderers striker was booked and then sent off against Armenia. But Trapattoni offered the names of Shane Long, Jon Walters and Simon Cox as reasons why Ireland should not worry. Cox deputised more than capably for Keane against Armenia.

Trapattoni's challenge is to lay the demons of Paris 2009, when his team were controversially knocked out of the World Cup play-off by France following that infamous Thierry Henry handball. There would also be a score to settle on a personal level for him were Ireland to draw Turkey, who are managed by Guus Hiddink, whose South Korea team beat his Italy in the last 16 of the 2002 World Cup.

Trapattoni suggested that Estonia could be Ireland's preferred opponents, given that they are physical and the other three countries are more technical and dangerous. But there is respect for everyone and no little confidence.

"We have only lost one group tie over my two qualifying campaigns. We have become increasingly compact, we make fewer mistakes and we have more options. Nobody should treat us lightly because we have proved ourselves with our results. We can achieve qualification."